1. Field of the Invention
This invention generally relates to a slide unit including a rail and a slider slidably mounted on the rail to provide a slide motion therebetween, and, in particular, to a statically pressure balanced slide unit in which a static pressure between the rail and the slider is maintained at a predetermined level.
2. Description of the Prior Art
A static pressure slide unit is well known in the art as described in the Japanese Utility Model Post-exam Pub. No. 63-48810. The slide unit described in this publication is schematically shown in FIGS. 7 and 8 of this application. As shown, the slide unit of this publication includes a table guide 30 having a rectangular cross section and a moving table 31 having a annular, rectangular cross section, which is fitted onto the guide 30 with a minute gap 32 therebetween. Pressurized air is introduced into the gap 32 via an air hose 33 and/or 34 so as to support the moving table 31 to which an external load is applied by means of the static pressure of the fluid in the gap 32.
However, in the above-described slide unit, the table guide 30 has four fixed, flat guide surfaces, i.e., flat top 35, bottom 37 and side 36, 38 surfaces, and the slider 31 also has four fixed, flat guide surfaces, i.e., flat top 39, bottom 41 and side 40, 42 surfaces, located opposite to the guide surfaces 35, 37, 36 and 38, respectively. As indicated by the arrows in FIG. 8, static pressure is applied to the gap 32 in a direction normal to each of the guide surfaces 35-38 and 39-42.
Under the condition, in order to maintain the static pressure in the gap 32 in a desired range, each of the guide surfaces 35-42 must be finished to a predetermined precision, which requires to use a high precision processing and assembly which, in turn, tends to push up the manufacturing cost. In addition, in the above-described slide unit, when a load F is applied to the table 31 as shown in FIG. 11, the table 31 is pushed downward so that a portion 32A of the gap 32 where load F has been applied decreases and its opposite portion 32B of the gap 32 increases. If the gap increases locally in this manner, there occurs an increased leakage of pressurized air, so that the static pressure inside the gap 32 cannot be maintained at a desired high level. In the case of FIG. 11, the table 31 is eventually shifted in a new position where the static pressure inside the gap 32 is balanced with the load F; however, the static pressure inside the gap 32 is not stabilized momentarily. It takes a finite period of time for the static pressure to settle and the table 31 to settle to its new balanced position. Such a period of time will be referred to as an unstable period here. The conventional slide unit notably has a relaively large unstable period. It is true that use may be made of an automatically adjustable static pressure bearing as well known in the art so as to alleviate or remove such a problem, but the overall structure tends to become bulky and the cost tends to increase.
If a load F is applied at one end of the table 31 as shown in FIG. 12, the table 31 becomes inclined with respect to the rail 30, thereby creating increased gap portions 32D and 32E and decreased gap portions 32C and 32F. In this case also, the table 31 is shifted in position until a balance is obtained between the load F and the static pressure inside the gap 32. Thus, there is also a relatively large unstable period.
The larger the static pressure inside the gap 32, the smaller the amount of displacement of the table 31 until its new stable position is reached. However, in order to set the static pressure inside the gap 32 at a higher level, the width of the gap 32 must be set smaller, which requires to process guide surfaces 35-38 and 39-42 more accurately, which, in turn, pushes up the manufacturing cost.
As set forth above, the conventional static pressure slide unit has disadvantages of relatively large unstable period, incapability to maintain a relatively high static pressure and low rigidity. Therefore, there has been a need to develop a novel static pressure slide unit which are free of the disadvantages of the prior art as described above.